The invention provides a resonator piece, a resonator, an oscillator and an electronic device that can prevent in advance defects from arising in the electrodes and whose frequency characteristics of the resonator piece and CI value are stable. The resonator piece can include a base portion in which a base portion electrode portion is formed, resonating arm portions, groove portions including groove electrode portions, side surface electrode portions, groove electrode-use connection electrode portions, and side surface electrode-use connection electrode portions 141. The width of the base portion side of openings of the groove portions is formed narrower than the width of other portions, with connection electrode disposition portions for disposing the groove electrode-use connection electrode portions or the side surface electrode-use connection electrode portions being formed near the narrowly formed openings of the groove portions, and the shapes of the openings of the groove portions are formed so as to be substantially symmetrical with respect to hypothetical lines disposed along the longitudinal direction at width-direction centers of the groove portions.